In light of dwindling oil reserves, geo-political uncertainties and increased concern over the environmental impacts of burning fossil fuels, there is a well recognised need to improve the efficiency of fuel use. This need is particularly apparent in relation to internal combustion engines, which are expected to power most of the world's transport needs over coming decades.
Additionally, increasingly stringent emissions requirements for pollutants such as unburnt hydrocarbons, carbon monoxide and nitrogen oxides (NOx) require internal combustion engines to burn fuel in conditions which mitigate against the formation of these pollutants.
It is thus necessary to have good control of the combustion process.
A great deal of research has been conducted into understanding and controlling the two principal combustion processes used in internal combustion engines, namely spark-ignition (SI) and compression-ignition (CI). In an SI engine, a spark ignites a compressed air/fuel mixture within a cylinder. The actual ignition takes over a period of time, as a flame front travels outwardly from the spark. In a CI ignition, fuel ignites as it is injected into the cylinder. Again, the ignition occurs over a period of time, being the time taken to complete the injection of fuel. In both SI and CI engines pressures and temperatures within the cylinder and on the piston form a gradient relative to the changes taking place over the time of ignition.
It has long been recognised that greater theoretical fuel efficiencies and/or reduction of engine emissions can be gained from an alternative combustion process, namely controlled auto-ignition or homogeneous charge compression ignition (HCCI). In a CAI combustion process, fuel is introduced into a cylinder and then compressed to a point where its temperature induces self-ignition. Ignition is typically induced at multiple sites, as the temperature and pressure are largely uniform. CAI combustion is generally distinguished by a significantly lower combustion temperature than SI or CI combustion, and as a consequence typically results in significantly lower NO emissions. Further, in comparison with CI combustion processes, CAI combustion processes have lower particulate matter emissions, thus reducing cost and complexity in the exhaust gas after-treatment system of such CI engines.
There are known limitations in the use of CAI combustion. Principal among these are excess rates of heat release and cylinder pressure rise during high engine load or speed, which can result in undesirable engine knocking. These factors cause an effective upper boundary of the speeds and/or loads where CAI combustion can be used. CAI combustion is therefore generally more suited to engine operation at lower speeds and/or loads.
The use of CAI combustion can also be problematic below an effective lower boundary of speed and load, particularly at engine idle. At or near idle it can be difficult to obtain sufficient heat to cause the necessary temperature rise for CAI conditions. This can result in a mis-fire within a cylinder.
Known CAI combustion processes are thus limited in their range of operation. In many engine applications this limited range is not sufficient, and therefore an engine must be configured to operate in CAI mode in a portion of its range and in SI or CI mode outside this range.
The limited range in which CAI combustion can be operated greatly reduces its commercial viability. Further, the need for a smooth transition between two combustion modes having different efficiencies and emission characteristics presents significant challenges. Resolution of these issues is largely dependent on the degree to which the CAI combustion process can be controlled.
An example of a typical range of operation for CAI combustion is shown in FIG. 1b. 
As CAI combustion is initiated by temperature, it is important to raise the temperature within the cylinder prior to combustion—that is, the temperature of the charge—in comparison with that required by SI and CI combustion processes. This is typically done by one of two means: heating the intake air and re-use or retention of exhaust gas.
Heating of intake air is generally not preferred for a number of reasons, including energy requirements, complexity of effective control and the need for a high compression ratio. Re-use or retention of exhaust gas is therefore preferred for current applications. In a CI combustion engine, the exhaust gas is typically re-used, by being re-circulated into the induction system through an appropriate valve. In a SI combustion engine, a portion of exhaust gas is typically retained in the cylinder for heating purposes, this being controlled through timing or profiling of induction and exhaust valve events.
The use of exhaust gas in this way presents particular challenges during transition between CAI and non-CAI modes of combustion. As noted above, one of the principal differences between modes is the temperature of exhaust gases. When these gases are being re-used or retained to provide an increased charge temperature, control of this to produce a desired in-cylinder temperature can be quite complex. Further, it will be apparent that the need for heat from exhaust gases typically means that an engine cannot be started in CAI combustion mode.
Many problems can arise if CAI combustion is not stable and well controlled. These include a risk of misfire, an increase in emissions, a reduction in efficiency, unacceptable levels of combustion noise and potential damage to the engine. Stability of the CAI combustion can be achieved by accurate control of the phasing (that is the timing of ignition) and the associated rate of heat release during the combustion process. Effective control of these parameters assists in operating the CAI combustion process at close to an optimum position, maximising the effective CAI-combustion operation range, and in providing effective transition between different combustion modes. Operation at an optimum position may relate to minimising of combustion noise, fuel consumption and/or engine exhaust emissions.
The key determinants of CAI-combustion operation are the temperature, pressure, concentration of reactants, movement of the reactants and the nature of the reactants. Of these, temperature is the most difficult parameter to control. In SI-combustion, control can be achieved by timing of the spark. In CI-combustion, control can be achieved through timing and apportionment of injection events. These options do not provide for adequate control of CAI-combustion. Further, as temperature and pressure may vary significantly from cylinder to cylinder and cycle to cycle it is preferable to both accurately measure these parameters and to control them on a per cycle basis within each cylinder.
Efforts have been made to achieve control of CAI combustion through developments in Engine Management System architecture, combustion sensing, and Engine Control Unit hardware and software. Developments in these areas have led to a greater ability to determine the cycle-by-cycle conditions in each cylinder, and therefore to analyse the nature (particularly the phase and rate) of the CAI combustion event. Even so, the ability to achieve effective control of this event is constrained by the capacity to alter in-cylinder temperature, pressure, composition and motion on an individual cylinder and per-cycle basis.
Adjustment of parameters such as intake air temperature, compression ratio and coolant temperature can be achieved in order to alter mean performance. These parameters generally cannot, however, be altered on a per-cylinder or per-cycle basis.
Temperature within a cylinder can be altered by altering the amount of exhaust gas retained or re-used. Adjustment of exhaust gas retention requires variable valve timing, which adds significant complexity to the engine design. Adjustment of exhaust gas re-use similarly requires complex porting arrangements.
The present invention seeks to provide a means of controlling CAI-combustion which is more effective than those outlined above in at least situations.